Fluidised bed incinerators

ABSTRACT

In a fluidized bed incinerator suitable for burning combustible refuse the refuse is deposited on the surface of one region of a bed of hot particulate refractory material contained in a vessel, the bed is fluidized in such a manner to cause the bed material to circulate so that the refuse deposited on the surface of the first region is drawn into the bed where the combustible content of the refuse is burnt and the non-combustible content of the refuse is displaced while submerged in the bed to a second region spaced horizontally from the first region and from which region the non-combustible content is removable from the bed.

United States Patent Muirhead et a1.

[54] FLUIDISED BED INCINERATORS [72] Inventors: Duncan Ronald Muirhead, Yorkshire; Douglas Allison Michell, Teesside, both of England [22] Filed: Feb. 4, 1971 [21] Appl. No.: 112,597

[52] US. Cl. ..ll0/8 R, 110/28 J [51] Int. Cl ..F23g 5/00 [58] Field of Search ..110/8 R, 18 R, 285

[56] References Cited UNITED STATES PATENTS 8/1968 Tada ..110/28X 12/1958 Godel ..ll0/28 Nov. 14, 1972 Shirai l 1 0/8 Muirhead 1 1 0/8 [57] ABSTRACT In a fluidized bed incinerator suitable for burning combustible refuse the refuse is deposited on the surface of one region of a bed of hot particulate refractory material contained in a vessel, the bed is fluidized in such a manner to cause the bed material to circulate so that the refuse deposited on the surface of the first region is drawn into the bed where the combustible content of the refuse is burnt and the non-combustible content of the refuse is displaced while submerged in the bed to a second region spaced horizontally from the first region and from which region the non-combustible content is removable from the bed.

9 Claims, 5 Drawing Figures 1 FLUIDISED BED INCINERATO'RS This invention relates to a method of disposing of combustible refuse in a fluidized bed incinerator and to such incinerators.

Fluidized bed incinerators for disposing of combustible refuse are known, for example from US. Pat. No. 3,397,657. This specification discloses a fluidized bed incinerator which comprises a vessel containing a fluidized bed of refractory particulate material, and a gas space above the fluidized bed and a smoke stack leading from the gas space in the vessel. In use the refuse is deposited onto the surface of the bed and the non-combustible content of the refuse is removed from an outlet positioned at the base of the bed. With such an incinerator the refuse is passed through part of the gas space onto the surface of the bed and there is a real danger that light material present in the refuse such as paper and dust is blown up into the smoke stack without being consumed in the bed. To prevent this from occurring it is necessary for the refuse to be surrounded by the bed material as soon as possible to prevent the lightweight material from being blown away from the bed.

According to a first aspect of the present invention in a method of disposing of combustible refuse, the refuse is deposited on the surface of one region of the bed of hot fluidized particulate refractory material and the material constituting the bed is circulated to cause the refuse to be drawn into the bed where the combustible content of the refuse is burnt and the non-combustible content of the refuse to be displaced to a second region of the bed displaced horizontally from the first region and from which second region the non-combustible content of the refuse is removeable from the bed.

Preferably the circulation of the material constituting the bed is brought about by fluidizing the bed at the second region to a greater degree of agitation than that at the first region and by blowing further air into the bed near the surface thereof at the second region with the flow of further air directed towards the first region.

The bed material is in a state of circulation such that refuse deposited on the surface of the bed is immediately drawn into the bed and covered by the bed material so that the lightweight material in the refuse cannot be blown through the gas space which is above the surface of the bed.

According to a second aspect of the present invention a fluidized bed incinerator suitable for burning combustible refuse comprises a vessel containing a bed of hot particulate refractory material, an opening in the vessel through which refuse is deposited onto the surface of one region of the bed, an opening in the vessel at a second region of the bed spaced horizontally from the first region through which opening the non-combustible content of the refuse is removeable from the bed, means for fluidizing the bed and for causing the bed material to circulate in such a manner that refuse deposited on the surface of the first region is drawn into the bed where the combustible content of the refuse is burnt and the non-combustible content of the refuse is displaced while submerged in the bed to the second reg Preferably the means for fluidizing the bed are arranged such that the bed material at the second region is fluidizable to a greater degree of agitation than at the first region and means are provided for blowing further air into the bed near the surface thereof at the second region with the flow of further air directed towards the first region.

To bring about differential fluidization of the bed between the first and second regions thereof the bed may be supported on a perforated plate structure with at least two separate air boxes positioned beneath the plate structure with one of the boxes beneath the first region of the bed and another of the boxes beneath the second region of the bed. Alternatively the bed may be supported on a perforated plate structure with a single air box positioned beneath the plate structure but the size of the apertures in the plate or the number of apertures per unit area of the plate is arranged so that a greater degree of fluidization is brought about at the second region than at the first region.

A fluidized bed incinerator in accordance with this invention ensures that combustible refuse deposited on the surface of the fluidized bed is drawn rapidly into the bed and that there is an adequate air supply to support complete combustion of the combustible content of the waste material.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a sectional side elevation of a fluidized bed incinerator,

FIG. 2 is a section on the line II-II of FIG. 1,

FIG. 3 is a section through a fluidized bed incinerator having a particular type of gas distributor,

FIG. 4 shows to an enlarged scale part of the gas distributor shown in FIG. 3 and FIG. 5 is a side elevation of part of a gas distributor of alternative form.

Referring to FIGS. 1 and 2, a fluidized bed incinerator comprises a vessel 10 having a metal wall with an inner lining of refractory material 11. An air distributor 12 is provided at the base of the vessel and a bed 13 of particulate refractory material such as sand is supported on the air distributor. The air distributor comprises a perforated metal plate 14 spaced above a second perforated plate 15 with separate air boxes 16 positioned beneath the plate 15. Each air box has a pipe 17 leading to it through which air under pressure is introduced into the air box and through the plates 14 and 15 into the bed material. In the upper part of the vessel and at one end thereof there is an opening 18 through which combustible refuse such as town refuse is introduced into the incinerator. A shredder, not shown, may be provided at the entrance to the opening 18 if desired to reduce the size of the combustible material entering the incinerator. A downwardly extending plate 19 forms a passage 20 below the opening 18 and material entering the bed passes through this passage. The plate 19 separates this passage from a gas space 21 in the vessel above the surface of the bed.

The plate 14 is inclined slightly to the base of the vessel, although this is not essential, and at the lower end of the plate in a side wall of the vessel there are a plurality of openings 22 each of which has a removeable cover 23. Above the openings 22, there are a plurality of pipes 24 extending through the side wall at an angle to the horizontal and each directed towards the lower end of the passage 20. Further air or secondary air, is introduced into the incinerator through the pipes. A shaped baffle plate projects into the incinerator vessel from the side wall at a position above the pipes 24. The plate 25 extends upwardly and then horizontally towards the lower end of the plate 19 leaving a space 26 between the plates 19 and 25. A further opening 27 in the upper wall of the vessel is provided above the space 26 and a chute connects with the opening 27. Below the openings 23 there is a screen 29 and material passing through the screen falls onto a conveyor which transports the material upwardly to the chute 28. A smoke stack 31 extends from the gas space 21 and is positioned above the plate 25 In use the fluidized bed material is initially heated to a temperature at which combustion of the combustible content of the refuse introduced into the incinerator is self-sustaining. To raise the temperature of the fluidized bed material, fuel is injected into the bed and burnt by means of electrically ignited burners. The bed is fluidized by injecting air under pressure through the pipes 17 into the base of the bed. The air passes through the distributor 12 and into the bed material. The pressure of the air supply to the pipes 17 across the base of the bed is not the same in each case and if P P or P indicate the air pressures supplied to the pipes 17 shown in FIG. 1 then P is less than P which in turn is less than P Pressure P is arranged to be such that the bed material in the vicinity of the openings 22 is in a violently agitated state while the bed material below the opening 18 is in a much quieter condition i.e., in a state of teeter. This differential air pressure and variation in agitation of the bed causes a flow of bed material around the incinerator in the direction indicated by the arrow 32. This flow of bed material is increased by injecting further air into the bed through the pipes 24. It will be seen that the air introduced through the pipes 24 imparts a horizontal component of force to the bed material which assists the circulation of the bed material. The quantity of bed material in the vessel is such that the surface of the bed is at a level with the bottom of the plate 19 so that the plate 25 defines the surface of the bed.

Once the bed has been brought to the required temperature and state of circulation the waste material is introduced in a continuous manner into the incinerator through the opening 18. The flow pattern of the bed material is such that the waste material introduced into the bed at the region below the passage 20 is rapidly drawn into the bed itself in order to ensure the most rapid heat transfer between the bed material and the refuse and an accelerating rate of combustion. The constant agitation of the bed material and the burning waste material ensures that boundary conditions are extremely short lived and that the extremely high degree of aeration obtained under the fluidized bed condition is fully effective. Light debris in the waste material, such as paper etc., does not float into the gas space 21. Combustion is complete within the bed and the need for a large gas or burner chamber above the bed does not arise. Smoke and gaseous material produced by the combustion passes through the opening 26 into the gas space 21 and out of the smoke stack 31. The sterile ashes resulting from incineration sink to the bottom of the fluidized bed and migrate towards the openings 22 in the side wall through which they can easily be removed. The non-combustible content of the refuse such as metal cans, glass and the like also migrate towards the openings 22. This migration is brought about essentially by the circulation of the bed material. If the base of the bed is inclined as shown in FIG. 1 this also assists in the migration but it is primarily the bed circulation which brings this about.

Ashes, non-combustible material and bed material are continuously withdrawn through the opening 22 and the fluidizing air cools the material. The bed material is separated from the ashes and non-combustible content by the screen 29 and the bed material is returned by the conveying means 30 to the chute 28 and back into the incinerator. In this way heat is recovered and returned by the re-cycled bed material and the net effect is equivalent to that of air pre-heating. The fluidizing air which is pre-heated by the ashes and bed material being withdrawn from the bed serves to classify the material being withdrawn. Light dust is blown back into the bed by the fluidizing air and is prevented from leaving the incinerator.

A gas distributor suitable for the base of the fluidized bed vessel is described in connection with FIGS. 3 to 5.

A fluidized bed vessel comprises a container 33 having a pair of side walls 34 and 35 and a base 36. A duct 37 extends through the base into the vessel and fluidizing gas, which may be air, enters the vessel through the duct. The particulate material 38 constituting the bed is contained in the vessel but has to be separated from the duct by a gas distributor 39. The reason for this is that the particulate material must not be allowed to fall into the duct and also the gas entering the vessel has to be distributed as evenly as possible over the full cross-sectional area of the base of the bed material.

The gas distributor 39 consists of a perforated plate 40 extending across the vessel and supporting the particulate material on its upper surface. The openings 41 in the plate may be of the order of A; to inch diameter. The plate is supported from the walls of the vessel on brackets 42 and the edges of the plate are sealed to the walls of the vessel. A second plate 43 which is also perforated is carried by the walls of the vessel and is positioned below the plate 40. In the arrangement shown in FIGS. 3 and 4 the plates 40 and 43 are parallel to one another. A plurality of spacer bars 44 are positioned between the plates 40 and 43 and serve to both separate the plates and also divide the space between them into sections. The spacer bars may be in the form of a grid. A fine mesh 44, conveniently of stainless steel, is attached as by spot welding to the underside of the plate 43. The openings of the mesh are of small cross-sectional area compared with the openings in plates 40 and 43 and the mesh together with the plate 43 constitute a high impedance to the gas flowing into the vessel through the duct(s) 37. The pressure drop across the lower plate and the mesh is high and the gas entering the vessel is distributed by the lower plate and the mesh substantially uniformly over the entire upper surface of the lower plate. The gas then passes through the openings in the plate 40 into the particulate material to fluidize it.

As the fluidized bed vessel is arranged with its longitudinal axis horizontal it may be desirable for the surface supporting the particulate material to be inclined to the horizontal and such an arrangement is shown in FIG. 5. The plate 43 and the mesh 44 are arranged substantially horizontal whilst the upper plate 40 is inclined to the horizontal. The spacer bars 45 are not of uniform depth as in the case with the arrangement shown in FIGS. 3 and 4.

Although the temperature of the bed material may be high the fine mesh is protected from the heat by the plates 40 and 43. Apart from providing a uniform distributor of fluidizing gas in the particulate material, the distributor has the further advantages that it is simple and relatively cheap to construct, and that the surface of the plate 40 is clear of obstructions and the particulate material can easily be removed from the vessel.

What we claim is:

1. A fluidized bed incinerator suitable for burning combustible refuse comprising a vessel containing a bed of particulate refractory material, said vessel defining a first opening above the surface of one region of the bed through which opening a mixture of combustible and non-combustible refuse is introduceable onto the surface of said first region and defining a second opening adjacent the base of the bed at a second region spaced horizontally from the first region and through which opening the non-combustible content of the refuse is'removable, means for fluidizing the bed in a non-uniform manner to cause a greater degree of agitation of the bed at the second region than at the first region thereby promoting circulation of the bed material in the vessel in the direction downwardly from the first region and towards the second region whereby the noncombustible content of the refuse is displaced while submerged in the bed from the first region to the second region.

2. A fluidized bed incinerator as claimed in claim 1 in which means are provided for blowing further air into the bed at the second region with the flow of the further air directed towards the surface of the first region.

3. A fluidized bed incinerator as claimed in claim 1 in which the bed is supported on a perforated plate structure with at least two separate air boxes positioned beneath the plate structure with one of the boxes beneath the first region of the bed and another of the I perforated plate for supporting the bed material, a

second perforated plate spaced below the first plate, and a finely apertured mesh attached to the underside of the second perforated plate.

6. A fluidized bed incinerator as claimed in claim 1 in which a baffle plate in the vessel limits the depth 'of the bed at the second region thereof and the baffle plate projects towards the first region of the bed.

7. A fluidized bed as claimed in claim 1 in which the vessel defines a gas space above the surface of the bed and a further opening defined by the vessel leads into the gas space with means for introducing particulate refractory material into the vessel through the opening leading into the gas space.

8. A fluidized bed as claimed in claim 7 in which said means include a screen for separating bed material from the non-combustible content of the refuse removed from the vessel and conveyor means for delivering said bed material to the opening.

9. A method of disposing of refuse which comprises the steps of providing a bed of hot particulate refractory material in an incinerator vessel defining a first opening above the surface of one region of the bed and a second opening adjacent the base of the bed at a second region spaced horizontally from the first region, fluidizing the bed in a non-uniform manner to cause a greater degree of agitation of the bed at the second re gion than at the first region thereby promoting circulation of the bed material in the vessel in the direction downwardly from the first region and towards the second region, introducing a mixture of combustible and non-combustible refuse through the first opening onto the surface of the bed, burning the combustible content of the refuse in the bed and withdrawing the non-combustible content of the refuse through said second opening.

UNITED STATES RATENT, OFFICE CERTIFICATE OF CORRECTION Patent No. 3,7 ,595 Dated November 1 1972 DUNCAN RONALD MUIRHEAD and Inventor(s) DOUGLAS ALLISON MICHELL It is certified that error appears in the above-identified patent v and that said Letters Patent are hereby corrected asshownbelow:

[73] Assignee: Power-Gas Limited, Stockton-on-Tees Tee sside, England Signed and sealed this 1st day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents :QRM P040550 uscoMM-Dc sows-ps9 U.S. GOVERNMENT FRINTING OFFICE 2 I959 0-356-334 

1. A fluidized bed incinerator suitable for burning combustible refuse comprising a vessel containing a bed of particulate refractory material, said vessel defining a first opening above the surface of one region of the bed through which opening a mixture of combuStible and non-combustible refuse is introduceable onto the surface of said first region and defining a second opening adjacent the base of the bed at a second region spaced horizontally from the first region and through which opening the non-combustible content of the refuse is removable, means for fluidizing the bed in a non-uniform manner to cause a greater degree of agitation of the bed at the second region than at the first region thereby promoting circulation of the bed material in the vessel in the direction downwardly from the first region and towards the second region whereby the non-combustible content of the refuse is displaced while submerged in the bed from the first region to the second region.
 2. A fluidized bed incinerator as claimed in claim 1 in which means are provided for blowing further air into the bed at the second region with the flow of the further air directed towards the surface of the first region.
 3. A fluidized bed incinerator as claimed in claim 1 in which the bed is supported on a perforated plate structure with at least two separate air boxes positioned beneath the plate structure with one of the boxes beneath the first region of the bed and another of the boxes beneath the second region of the bed.
 4. A fluidized bed incinerator as claimed in claim 3 in which the perforated plate is inclined downwardly from beneath the first region to beneath the second region.
 5. A fluidized bed incinerator as claimed in claim 3 in which the perforated plate structure comprises a first perforated plate for supporting the bed material, a second perforated plate spaced below the first plate, and a finely apertured mesh attached to the underside of the second perforated plate.
 6. A fluidized bed incinerator as claimed in claim 1 in which a baffle plate in the vessel limits the depth of the bed at the second region thereof and the baffle plate projects towards the first region of the bed.
 7. A fluidized bed as claimed in claim 1 in which the vessel defines a gas space above the surface of the bed and a further opening defined by the vessel leads into the gas space with means for introducing particulate refractory material into the vessel through the opening leading into the gas space.
 8. A fluidized bed as claimed in claim 7 in which said means include a screen for separating bed material from the non-combustible content of the refuse removed from the vessel and conveyor means for delivering said bed material to the opening.
 9. A method of disposing of refuse which comprises the steps of providing a bed of hot particulate refractory material in an incinerator vessel defining a first opening above the surface of one region of the bed and a second opening adjacent the base of the bed at a second region spaced horizontally from the first region, fluidizing the bed in a non-uniform manner to cause a greater degree of agitation of the bed at the second region than at the first region thereby promoting circulation of the bed material in the vessel in the direction downwardly from the first region and towards the second region, introducing a mixture of combustible and non-combustible refuse through the first opening onto the surface of the bed, burning the combustible content of the refuse in the bed and withdrawing the non-combustible content of the refuse through said second opening. 